Ethernet™ is a data link-layer protocol (Layer 2 in the Open Systems Interface [OSI] model), which is defined by IEEE standard 802.3. In Ethernet networks, the key functions of the data link layer (which is referred to hereinafter for brevity simply as the “link layer”) are performed by the medium access control (MAC) sublayer, and the terms “link layer” and “MAC layer” are generally used interchangeably.
Ethernet Layer-2 networks are commonly integrated as subnets of network-layer (Layer 3) Internet Protocol (IP) networks. A subnet (short for subnetwork) is a logical subdivision of a Layer-3 network. Network ports of nodes within a given subnet share the same Layer-3 network address prefix. For example, in IP networks, the ports in each subnet share the same most-significant bit-group in their IP address. Typically, the logical subdivision of a Layer-3 network into subnets reflects the underlying physical division of the network into Layer-2 local area networks. The subnets are connected to one another by routers, which forward packets on the basis of their Layer-3 (IP) destination addresses, while within a given subnet packets are forwarded among ports by Layer-2 switches or bridges. These Layer-2 devices operate in accordance with the applicable Layer-2 protocol and forward packets within the subnet according to the Layer-2 destination address, such as the Ethernet MAC address.
Both Ethernet and IP standards define packet header fields that can be used to specify different quality of service (QoS) levels, in the form of different priority values that are assigned to different packets. For example, the IP header includes a service type field. According to the original DARPA Internet Protocol specification, published by the Internet Engineer Task Force (IETF) as Request for Comments (RFC) 791, the service type field may contain three bits specifying the type of service (TOS) of the packet. As a more recent alternative, IETF RFC 2474 provides a six-bit differentiated services code point (DSCP) in the IP header. For Ethernet, the IEEE 802.1p standard specifies a three-bit priority code point (PCP) field in the virtual local area network (VLAN) tag of Ethernet frames (as defined by the IEEE 802.1Q standard). The PCP field of the VLAN tag can thus be used to specify eight different QoS levels (or equivalently, classes of service).